Moxa PT-7528-20MSC-4TX-4GSFP-HV-HV [68/116] Using multicast filtering

PT-7528 Series Advanced Settings

4-48

NOTE

Most Moxa managed switches have a maximum of 64 VLAN settings. However, the IKS

G6524/G6824 and

ICS

-G7000 series support up to 256 VLAN settings.

Using Multicast Filtering

Multicast filtering improves the performance of networks that carry multicast traffic. This section explains

multicasts, multicast filtering, and how multicast filtering can be implemented on your Moxa switch.

The Concept of Multicast Filtering

What is an IP Multicast?

A multicast is a packet sent by one host to multiple hosts. Only those hosts that belong to a specific multicast

group will receive the multicast. If the network is set up correctly, a multicast can only be sent to an end-station

or a subset of end-stations on a LAN or VLAN that belong to the multicast group. Multicast group members can

be distributed across multiple subnets, so that multicast transmissions can occur within a campus LAN or over

a WAN. In addition, networks that support IP multicast send only one copy of the desired information across the

network until the delivery path that reaches group members diverges. To make more efficient use of network

bandwidth, it is only at these points that multicast packets are duplicated and forwarded. A multicast packet

has a multicast group address in the destination address field of the packet’s IP header.

Benefits of Multicast

The benefits of using IP multicast are:

• It uses the most efficient, sensible method to deliver the same information to many receivers with only one

transmission.

• It reduces the load on the source (for example, a server) since it will not need to produce several copies of

the same data.

• It makes efficient use of network bandwidth and scales well as the number of multicast group members

increases.

• Works with other IP protocols and services, such as Quality of Service (QoS).

Multicast transmission makes more sense and is more efficient than unicast transmission for some applications.

For example, multicasts are often used for video-conferencing, since high volumes of traffic must be sent to

several end-stations at the same time, but where broadcasting the traffic to all end-stations would cause a

substantial reduction in network performance. Furthermore, several industrial automation protocols, such as

Allen-Bradley, EtherNet/IP, Siemens Profibus, and Foundation Fieldbus HSE (High Speed Ethernet), use

multicast. These industrial Ethernet protocols use publisher/subscriber communications models by

multicasting packets that could flood a network with heavy traffic. IGMP Snooping is used to prune multicast

traffic so that it travels only to those end destinations that require the traffic, reducing the amount of traffic on

the Ethernet LAN.

Multicast Filtering

Multicast filtering ensures that only end-stations that have joined certain groups receive multicast traffic. With

multicast filtering, network devices only forward multicast traffic to the ports that are connected to registered

end-stations. The following two figures illustrate how a network behaves without multicast filtering, and with

multicast filtering.